Hydroformylation of an olefin to produce a formyl-substituted derivative of the olefin is now well-known in the art as an economically attractive method for producing, in particular, the aldehydes which are the primary intermediates in the manufacture of, for example, alkanols such as n-butanol and the corresponding alkanoic acids. Also important are such end products as 2-ethylhexanol, which is formed from n-butyraldehyde by a sequence of steps including aldoling, dehydration, and hydrogenation by methods which are well-established in the art.
While hydroformylation processes using cobalt carbonyl as the major component of the catalyst have been known and used for many years, systems in which the catalyst comprises rhodium hydrido carbonyl complexed with an organic ligand have been developed more recently and are now favored over the older technology for several reasons including the fact that they can be used under relatively mild reaction conditions and also, of very great importance, the fact that the rhodium-catalyzed systems can be controlled so as to yield a product in which the normal isomer of the aldehyde predominates over the branched-chain isomer to a greater extent than has normally been obtained heretofore when using the older methods. It will be understood in this connection that for most industrial purposes, including use as a raw material for production of the corresponding alkanoic acids (by catalytic oxidation of the aldehyde) and also for the production of higher molecular weight alcohol derivatives (as by aldoling etc.), the normal aldehyde is strongly preferred over the branched-chain isomer. In the case of the butyraldehydes, for example, n-butyraldehyde finds a ready and expanding market whereas isobutyraldehyde has fewer uses and is considered an undesirable by-product. Similarly, in the case of longer-chain aldehydes such as heptaldehyde, the normal isomers can be used to produce high-quality ester-type synthetic lubricants, while the properties of the corresponding branched-chain isomers are such that they have little value for such purposes.
Use of the rhodium-containing catalyst systems results in the attainment of an improved normal:iso ratio in the aldehyde products formed in these processes (as compared with the cobalt-based systems), but formation of the branched-chain isomer continues to be a significant economic drawback. By controlling such parameters as carbon monoxide partial pressure, carbon monoxide:hydrogen ratio, etc. it is possible to influence the product distribution somewhat in a favorable direction. A very significant process parameter is also the ratio of ligand to rhodium in the catalyst mixture, it having been discovered that the normal:iso ratio in the product increases with increasing ligand:rhodium ratio. For example, phosphine-type ligands, including specifically and for example triphenylphosphine, are customarily employed in rhodium-catalyzed hydroformylation systems in proportions such that the ratio of phosphorus to rhodium is at least about 10:1, ranging on upwardly to as much as 1000:1. Ratios lower than about 2:1 have been found to be distinctly unsatisfactory. As the phosphorus:rhodium ratio is increased in the systems employing the previously-recognized ligands such as triphenylphosphine, there is a gradual improvement in the normal:iso ratio in the product aldehydes indicative of an equilibrium-type reaction. Thus, normal practice is to use a substantial excess of ligand on the basis of judgment and various experience factors including, for example, practical observation of the rapidity of catalyst deactivation observed with various ligand:rhodium ratios.
The state of the existing art in the field of hydroformylation of olefins using as catalysts rhodium hydrido carbonyl complexed with organic ligands, including particularly phosphines and also phosphites, is exemplified by U.S. Pat. No. 3,239,566 to Slaugh and also U.S. Pat. No. 3,527,809 to Pruett et al. as well as U.S. Pat. No. 3,511,880 to Booth. These patentees describe hydroformylation processes in which, by using a complexed Group VIII noble metal, and particularly rhodium, the high pressures required in the cobalt carbonyl-catalyzed reaction systems are avoided while attractive ratios of normal aldehyde to branched-chain aldehyde are also obtained in the products. The ligands employed, however, as exemplified by triphenylphosphine, are used in a substantial excess. That is, as will be seen from examining the disclosures of these and similar related prior-art references, at least about 2 moles of the ligand are used per atom of rhodium. Also, continuing research in these reactions has indicated that, with these prior-art ligands, there is no ligand:rhodium ratio (up to compositions in which the entirety of the reaction medium comprises ligand) above which the addition of more of the ligand ceases to have an effect on product distribution.
More recently it has been discovered, as disclosed in Belgian Pat. No. 840,906 (Oct. 20, 1976), that the nature of the ligand in these reaction systems is a more significant factor than has been recognized heretofore. More particularly, it has been discovered that certain bidentate ligands which are derivatives of ferrocene are capable, in catalytic complexes with rhodium, of yielding hydroformylation product mixtures in which there is an unusually high ratio of normal isomer to branched-chain isomer without the requirement of employing a high ratio of ligand to rhodium in the catalyst. Furthermore, with these ferrocene-based ligands (which include specifically diphosphino-substituted ferrocenes) there is little need for maintaining in the reaction zone more than about 1.5 moles of the ferrocene derivative per atom of rhodium (a phosphorus:rhodium mole ratio of 3.0:1). Insofar as the teachings of Belgian No. 840906 are concerned, however, the ligands reported therein require the presence of the ferrocene moiety.
The use of bidentate diphosphino ligands is also disclosed in British Pat. No. 1,402,832, to Pino, wherein it is disclosed that asymmetric hydroformylation of olefinically unsaturated prochiral compounds can be accomplished by carrying out the hydroformylation in the presence of an optically active diphosphino compound, with the recommended diphosphino compounds including certain bis-(diphenylphosphinomethyl) compounds. The thrust of British Pat. No. 1,402,832 is in the direction of obtaining optically active aldehyde products. There is no teaching of any benefit from using optically inactive ligands of this general type, nor of any advantage of such compounds, whether optically active or not, as ligands in relation to such factors as normal:iso aldehyde distribution in the reaction product.
It is an object of the present invention to provide a family of bidentate ligands for use in rhodium-catalyzed hydroformylation processes which do not require optical activity for their efficacy and which need not be employed, in the reaction system, at the high ligand:rhodium ratios characteristic of the related prior art. It is another object to provide an improved hydroformylation process for converting an ethylenically-unsaturated raw material to a formyl-substituted derivative thereof wherein the ratio of normal aldehyde to branched-chain aldehyde in the reaction product is of a high, economically-attractive level without the necessity of using stringent reaction conditions nor a high excess of ligand. Other objects will be apparent from the following detailed description.